Gigabit-Capable Passive Optical Network (GPON) technology is an important technical branch in the Passive Optical Network (PON) family. Similar to other PON technologies, GPON is a passive optical access technology adopting point-to-multipoint topology.
FIG. 1 is a schematic diagram of a topology of a GPON system according to the relevant art. As shown in FIG. 1, the GPON consists of an Optical Line Terminal (OLT) on the office center side, an Optical Network Units (ONU) on the user side, and an Optical Distribution Network (ODN), and usually adopts a point-to-multipoint network structure. The ODN consists of passive optical devices such as single mode fibers, an optical splitter and an optical connector, and provides optical transmission medium for a physical connection between the OLT and the ONU. The range of the downstream wavelength band of GPON is 1480 nm to 1500 nm, and the range of the upstream wavelength band is 1290 nm to 1330 nm; and the downstream rate of GPON is 2.5 Gbit/S, and the upstream rate is 1.25 Gbit/S.
With the development of Internet technology and the increasing requirements of large bandwidth, the GPON-based next generation PON technology is developed rapidly, and it is commonly believed that the XG-PON1 technology is a next generation PON technology that may be adopted in the future. The downstream rate of XG-PON1 technology is 10 G, and the upstream rate is 2.5 G, which may satisfy the bandwidth needs of resident users. The range of the downstream wavelength band of XG-PON1 system is 1575 nm to 1580 nm, and the range of the upstream wavelength band is 1260 nm to 1280 nm. The XG-PON1 system reuses the ODN of GPON system, and the downstream signals of XG-PON1 (a Gigabit-Capable passive optical network with a rate of 10 Gbit/S) system and GPON system are combined by a WDM device and transmitted to an ONU of XG-PON1 and an ONU of GPON via the same ODN. Each of the ONU of XG-PON1 and the ONU of GPON is provided with a filter, the filter of the ONU of XG-PON1 filtering out the downstream signals of GPON, the filter of the ONU of GPON filtering out the downstream signals of XG-PON1.
At present, operators make a request for transmission of wireless signals, high definition television signals, etc. in a PON system, which requires to further increase the transmission bandwidth of PON system. In order to meet the above requirements, the next generation PON technology, i.e., NGPON2 technology, with a downstream transmission rate being at least 40 Gbit/s and with an upstream transmission rate being at least 20 Gbit/s, is proposed. In order to ensure that the service of current users in a network is unaffected and to protect the ODN that is invested and established by an operator, it is needed to coexist the NGPON2 technology with the GPON and XG-PON1 systems on the existing ODN, or only coexist the NGPON2 technology with the GPON or XGPON1 system on the existing ODN. In order to solve the above problem, it is in urgent need of determining the downstream wavelength band value of downstream signals and the upstream wavelength band value of upstream signals transmitted by the NGPON2 system. The improper setting of the wavelength values of NGPON2 will cause the Raman effect in the optical fibers, thus the degradation of signal-to-noise ratio, carrier-to-noise ratio and optical power budget of the PON system or analog television signals transmitted in the ODN is caused.